It is often necessary in the petroleum industry to determine the level of a liquid standing in a casing of a well bore. Prior art systems have been utilized for carrying out such measurements. Representative of the prior art are U.S. Pat. Nos. 2,047,974; 2,156,519 and 3,316,997. In each case a pressure wave is introduced into the casing at or near the earth's surface. The pressure wave travels downwardly through the annulus gas between the well tubing and the casing. The sound wave is reflected back to the top of the casing from the gas-liquid interface. In addition, reflections occur at other points of acoustic impedance discontinuity in the bore hole such as from the collars utilized in connecting sections of tubing in the well bore.
Where tubing is standing in the casing and the lengths of the tubing joint are known, depth information may be determined by counting each of the small pressure wave reflections from the tubing collars and then locating the liquid level reflection in relation to the tubing collar reflections. Sometimes, the returning pressure waves or echoes are detected and recorded as a function of time. The collar reflections may be missing or not distinct. The depth or the level of the liquid can then be calculated from the elapsed time and the acoustic wave velocity. In the above-referenced patents, the pressure wave is generated by releasing a high pressure gas from the chamber by means of a valve or by utilization of an explosive charge such as a blank shotgun shell.
The present invention is directed towards a much improved gas gun assembly and particularly to structure which can produce a distinctive high efficiency energy impulse at low cost. The construction dealing with the detector system is so arranged that the gas gun assembly is substantially failsafe providing long life and repeatable excellent results. The use of blank cartridges, such as shotgun shells, has the disadvantage of discharging burning particles of black powder which is a fire hazard and which could cause injury to personnel. The gas gun assembly of the present invention is safer than prior systems. Even though it is small in size, it will give records greatly improved over those obtainable with prior art systems. Further, it can be operated either in the explosion or implosion mode.